Since the discovery of single-walled carbon nanotubes (SWNTs), there has been interest in exploiting them. Iijima, S., Helical Microtubules of Graphitic Carbon, Nature 354 (6348), 56-58, (1991). Because of their unique characteristics, SWNTs hold potential for use in many industries. Dresselhaus, M. S. et al., Philos. Trans. R. Soc. London, 362, 2065, (2004). On the molecular level, carbon nanotubes (CNTs) are the strongest molecules known. Li, Q., et al., Adv. Mater., 18, 3160, (2006). CNTs are a hundred times stronger than high-strength steel, at a tenth of the weight and have a Young's modulus approximately five times that of hardened steel. Haskins, R. S., et al., J. Chem. Phys. 127, 074708, (2007).
Contingent on chirality, SWNTs can be either metallic or semiconducting. Metallic nanotubes may conduct electric current densities 1000 times greater than copper can. Hong, S. and S. Myung, Nat. Nanotechnol. 2, 207, (2007). CNTs are excellent thermal conductors. Biercuk et al., Appl. Phys. Lett. 80, 2767, (2002). These unique physiochemical properties hold promise for use in structural, mechanical, chemical, and electrical applications. However, low aqueous solubility and intrinsic difficulty in proficiently aligning CNTs have limited their use.
Low solubility may be overcome by covalent chemical modifications. These processes may adversely affect physical and electrochemical properties of CNTs. Haung, W., et al., Langmuir 19, 7084, (2003); Liu, P., Eur. Polym. J., 41, 2693, (2005). Biological molecules, bonding non-covalently, offer less invasive modification and are used both to separate and purify CNTs. Zheng et al. have demonstrated the intrinsic ability of single-stranded DNA (ssDNA) to bind and disperse SWNT bundles in aqueous solution. Zheng, M. et al., Nat. Mater. 2, 338, (2003a). One theory is that π bonds are formed between the graphene surface and the hydrophobic base pairs of the DNA, resulting in a helical wrapping of ssDNA around the CNT. The hydrophilic phosphate backbone of the DNA remains exposed, causing electrostatic stabilization in water. After being solubilized, SWNTs can be separated and purified on the basis of size and chirality by ion exchange chromatography or gradient centrifugation. Zheng (2003a); Arnold, M. S. et al., Nat. Nanotechnol. 1, 60, (2006); Huang, X., et al., Anal. Chem. 77, 6225, (2005); Lustig, S. R., et al., J. Phys. Chem. B109, 2559, (2005); Zheng, M., et al., Science 302, 1545, (2003). Creating pure soluble SWNTs is necessary in extending their potential.
Few efforts have demonstrated how to assemble CNTs, particularly using biological means. The use of DNA to assemble CNTs into nano-devices has attracted attention because of the recognition specificity of the DNA molecule. RecA-based motifs used DNA to localize CNTs to form nano-wires and transistors. Hazani, M., et al., Chem. Phys. Lett. 391, 389, (2004); Keren, K., et al., Science 302, 1380, (2003). Additionally, nucleic acid hybridizations joined DNA-functionalized CNTs having complementary sequences. Li, Y., et al., Angew. Chem., Int. Ed. 46, 7481, (2007); Li, S., et al., J. Am. Chem. Soc. 127, 14, (2005). Exploiting biologically-based assembly motifs provides for building higher-order nanostructures with precise control.
Select embodiments of the present invention comprise a novel method for assembling DNA-functionalized SWNTs by phosphodiester bonding catalyzed by ssDNA-ligase to form macroscopic CNT aggregates. Exploiting biological means such as these to direct assemble CNT-based nanostructures allows for scaling to manufacture.